Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF T~E INVENTION
The blind rivet of the instant invention is an
improvement on the composite rivet disclosed in UO S.
Patent No. 4,478,544 issued October 23, 1984, for
Composite Rivet, and assigned to the assignee hereof.
Carbon fiber reinforced materials are no~
widely used in the aircraft industry for airframe
structural components. However, the use of carbon fiber
reinforced resins in blind rivets has been limited by
the difficulty of properly forming the blind head. One
characteristic of composite materials utilizing carbon
ibers is that the material often exhibits a rough
finish after reforming due to protrusion or breakage of
the carbon fibers. While the structural integrity of
the material may not be compromised, the end product is
abrasive and aesthetically unsatisfactory. When such
material is employed in the reformable head of a blind
rivet, the problem becomes acute since the blind head is
exposed on the rear surface of ~he workpiece.
Another problem that must be addressed is that
plastic cold flow of the rivet tends to reduce clamp-up
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forces on a workpiece. Thus, pretensioning of the rivet
is highly desirable to offset ultimate relaxation of the
tensile forces.
In the '544 patent, a high tensile strength,
low modulus tubular sheath encapsulates the composite
core of the rivet taught therein so as to prevent the
protrusion of reinforcement fibers therefrom subsequent
to blind head formation. However, the sheath relies
upon the controlled and proper deformation of the
underlying composite core to produce a blind head of a
configuration suitable for use. It will be appreciated
that the uncontrolled, nonuniform radial expansion of
the composite core during blind head formation may
displace the sheath so as to render its protection
unavailing.
Additionally, the sheath as taught in the '544
patent provides no radially inward bias on the rivet's
composite core in order to prevent cold flow thereof,
nor does the sheath provide an axial bias against the
anvil to pretension the rivet and, hence, minimize the
reduction of clamp-up force upon plastic cold flow of
the shear portion of the rivet's composite core.
SUMMARY OF THE INVENTION
The aforesaid problems are solved by the ~ivet
of the present invention by encapsulating the shear and
blind head forming portions of the rivet's tubular
composite body in a metallic sheath. The sheath is
provided with a thin-walled section proximate to the
outer surface of the blind side of the workpiece, and a
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thick-walled section which radially engages with a
radial shoulder of the anvil of the mandrel extending
therethrough and which axially engages with an axially-
disposed surface of the anvil subsequent to initial
axial displacement of the mandrel.
Upon commencement of the heading op~ration,
the sheath operates in association with the radial
shoulder of the anvil to form a closed pressure chamber
wherein the head-~orming portion of the rivet's
composite core, which is softened due to the application
of heat, is pressurized upon initial axial movement of
the mandrel. The pressurized composite core in turn
hydrostatically radially expands the thin-wall section
of the sheath in a uniform, symmetrical manner. The
thick-walled section of the sheath then engages with the
axially-disposed surface of the anvil, whereafter the
sheath is axially compressed between the anvil. and an
axially-disposed surface on the tubular composite rivet
body upon subsequent axial movement of the mandrel.
The sheath is thus controllably buckled
radially outwardly, initially hydrostatically by the
radial expansion of the rivet's composite core, and
thereafter by the combination of composite core
hydrostatics and the axial compression of the sheath.
It is significant that the initial uniform, symmetrical
hydrostatic deformation o the sheath encourages further
uniform, symmetrical buckling of the sheath upon the
subsequent mechanical axial compression thereof by the
anvil. Thus, a properly-formed blind head is more
reliably obtained. Moreover/ the sheath is maintained
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about the head-forming portion of the rivet of the
instant invention throughout the heading operation,
whereby the projection of the reinforcement fibers
therefrom is prevented.
Additionally, since the head-forming portion
of the rivet's composite core is maintained in a
pressurized state both during and after blind head
formation by the sheath, greater core homogeneity, with
all its attendant benefits, is achieved. Additionally,
the inal configuration of the sheath aids in the
maintenance of workpiece clamp-up forces by elastically
exert~ng an a~ial bias against the anvil and the
workpieces, while preventing plastic cold flow of the
head forming portion of the rivet's tubular composi~e
body subsequent to rivet heading by exerting a radially
inward bias thereon.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view, partially in
cross section, of an exemplary embodiment of the blind
rivet of the instant invention;
FIG. 2 is a view of the rivet of FIG. 1 after
tensioning of the rivet mandrel to form a blind head o~
the rivet; ~ ~
FIG. 3 is a view of the rivet of ~IG. 1 after
initial hydrostatic radial expansion of the thin-walled
section of the sheath thereof, immediately prior to the
axial engagement of the thick-walled section thereof
with an axially-disposed surface on the anvil; and
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FIG. 4 is an elevation view of an alternate
embodiment o~ the blind rivet of the instant invent.ion.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS OF THE INVENTION
-
As best seen in FIG. 1 of the drawings, a
composite blind rivet 10 in accordance with an exemplary
constructed embodiment of the instant invention
comprises a tubular composite body 11, preferably a
preform comprised of a carbon fiber-reinforced "B"-stage
thermoset resin, having a preformed head portion 12, a
shear portion 14 connected to the preformed head
portion 12 by a radially extending shoulder 15, and a
blind head forming portion 16. The head forming
portion 16 of the preform 11 is additionally provided
with an end portion 17 of relatively smaller diameter
which is connected thereto by a tapered intermediate
portion, such as the truncated conical section l9
illustrated in FIG. 1. ~ mandrel 18 is disposed
centrally of the preform 11 and has an anvil 20 at one
end thereof overlying the blind head forming portion 16
of the preform 11 and a tensioning stem 22 at the other
end thereof. The mandrel 18 is journaled in a
complementary aperture 24 that extends through the
preformed head, shear and head forming portions 12, 14,
and 16, respec~ively, of the preform 11. The shear
portion 14 of the rivet 10 is coextensive with the
cumulative thickness of a pair of workpieces 30 and 32.
As seen in FIG. 2 of the drawings, the
mandrel 18 is adapted to be pulled, aftér heating of the
preform 11, by a tool (not shown) of conventional design
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resulting in deformation of head forming portion 16 of
the rivet lO into the mushroom configuration shown.
In accordance with the instant invention, the
head forming portion 16 of the rivet 10 is encapsulated
by a unitary cylindriral sheath 34, preferably formed of
a ductile metal such as titanium, which is buckled
radially outwardly to the configuration of the blind
head upon the formation thereof, thereby preventing the
protrusion of the carbon fibers 36 that are impregnated
or encapsulated in the resin matrix thereof. The
sheath 34, which is of uniform external diameter prior
to deformation, has a thick-walled section 37 on the
blind end 41 thereof underlying the anvil 20; a
relatively thin-walled section 39 on the end 42 thereof
extending through, and radially aligned with, the
apertures in workpieces 30 and 32 and abuttin~ radial
shoulder 15 of the preform 11; and an intermediate wall
section 38 connectin~ the thick- and thin-walled
sections 37 and 39 and shaped so as to complementarily
engage with the intermediate portion 19 of the
preform ll.
A radial shoulder 50 on the anvil 20 radially
engages with the inner surface 52 of the ~hick-walled
section 37 of the sheath 34 to provide a closed pressure
chamber 54 wherefrom the head forming portion 16 of the
rivet's composite core 11 encapsulated by the sheath 34
cannot escape. Thus, during initial axial displacement
of the anvil 20 relative to the preformed head and shear
portions 12 and 14 of the rivet 10, the inner axially-
disposed surface 56 of the anvil 20 engages with and
hydrostatically compresses the head-forming portion 16
of the rivet's composite core ll. The ~esultant
pressurized composite core 11 in turn exerts a uniformly
distributed, radially outward hydrostatic force on the
sheath 34.
The part of the thin-walled section 39 of the
sheath 34 indicated generally at 58 thus begins to yield
plastically to the hydrostatic pressure of the composite
core 11 and is uniformly and symmetrically radially
outwardly expanded thereby, as illustrated in FI~. 3 of
the drawings. The initiation of radially outward
buckling of the sheath 34 is thus precisely controlled
by the hydrostatic pressure induced in the composite
core 11 and the presence of thin-wall section 58. It is
noted that the thick-walled section 37 of the sheath 34
preferably commences well above the workpiece 30,
thereby providing a sufficient length of thin walled
section 58 to accommodate radial deflection thereof and,
thus, helping define the ultimate truncated conical
cross-section of the blind head forming porti.on 35 of
the rivet.
It is significant that the composite core 11
is fully contained by the sheath 34 during blind head
formation so that the sheath 34 is initially e~panded
radially outwardly by the resu~tant hydrostatic pressure
of the composite core 34. To that end, it is desirable
for the sheath 34 to extend substantially ~hrough the
entire thickness of workpieces 30 and 32, as illustrated
in the drawings.
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The resultant uniformly radially expanded
thin-walled section 58 of the sheath 34 thereafter
encourages further uniform and symmetrical radially
outward buckling of the sheath 34 as by mechanical axial
compression thereof upon engagement o~ the blind end 41
of the sheath 34 with a complementary axially-disposed
surface 60 of the anvil 20 Thus, upon further axial
movement of the mandrel 18, the axially-disposed
surface 60 engages with the blind end 41 of the
sheath 34, whereafter the sheath 34 is mechanically
axially compressed between anvil surface 60 and the
axially-disposed radial shoulder 15 of the preform 11.
It is noted that, as the compressive load during such
further deformation of the sheath 34 is borne by radial
shoulder 15 of the preform 11, damage to the
workpieces 30 and 32 by the so-called "cookie-cutter
effect" is avoided by the rivet 10 of the instant
invention.
From the foregoing it should be apparent that
a smooth exterior surface is presented on the blind head
which is not abrasive and is aesthetically pleasing
Moreover, subsequent to blind head formation on the
rivet 10, the sheath 34 maintains a radially inward
pressure on the composite core 11 during the curing
thereof, whereby greater homogeneity of the composite
core 11 is achieved, thereby ensuring posi~ive
impregnation of the reinforcement fibers therein, and
producing greater uniformity in the physical properties
thereof. Moreover, the sheath 3b tends to maintain the
cured composite core 11 in a compacted state, thereby
preventing undesirable radial plastic cold flow thereof
during use which would otherwise result in a reduction
of clamp-up force.
Additionally, the sheath 34 elastically biases
the anvil 20 axially from the workpieces 30 and 32
subsequent to hlind head formation, whereby clamp-up
force is maintained notwithstanding nominal axial
plastic cold-flow of the shear portion 14 of the rivet's
composite core 11 during use.
In accordance with another feature of the
instant invention, the mandrel 18 is provided with an
annular groove 40 in the stem 22 thereof into which
resin flows thereby tensioning the mandrel and locking
it within the head forming portion 16 of the rivet 10.
A breakoff groove 44 is provided in the stem 22 to
facilitate removal of excess stem portion after setting
of the rivet 10. It is noted that pressurization of the
composite core 11 within the closed pressure chamber 54
comprised of sheath 34 and anvil 20 further ensures that
groove 40 is completely filled with resin during
formation of blind head 35 on the rivet 10, as
illustrated in FIG. 2.
An alternate embodiment 64 of the rivet of the
instant invention is illustrated in FIG. 4, wherein the
tapered intermediate portion 66 of the rivet preform 68
connecting the end 70 of the head forming portion 72
thereof with the shear portion 74 thereof, and the
corresponding intermediate wall section 76 of the
sheath 78 encompassing the preform 68, are characterized
b~ a more gentle taper. It is to be noted, however,
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that the cylindrical inner surface 80 of the thick-
walled section 82 of the sheath 78 extends axially
sufficiently to ensure radial contact between the
surface 80 and the radial shoulder 84 of the anvil 86
throughout initial hydrostatic expansion of the thin-
walled section 88 of the sheath 78. It is noted that,
with the more gentle taper employed with the alternate
embodiment 64, initial hydrostatic buckling of the
sheath 78 will occur along the thin-walled section 88
thereof at the point ~hereon indicated generally by
reference numeral 90, in close pro~imity to
workpiece 92.
While the preferred embodiment of the
invention has been disclosed, it should be appreciated
that the invention is susceptible of modification
without departing from the scope of the following
claims.
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